The invention relates to antistatic or electrically conductive thermoset polyurethanes, in particular casting elastomers and casting resins, which contain conductive carbon nanotubes, a process for producing them and their use for producing, for example, coatings, rolls, rollers and wheels.
Polyurethanes have been known for a long time and are notable for their great variety. An overview of polyurethanes, their properties and uses is given, for example, in Kunststofthandbuch, Volume 7, Polyurethane, 3rd revised edition, Volume 193, edited by Prof. Dr. G. W. Becker and Prof. Dr. D. Braun (Carl-Hanser-Verlag, Munich, Vienna).
In addition to flexible and rigid foams, unfoamed, noncellular polyurethanes, e.g. casting elastomers and casting resins, are also of interest. Particularly noncellular polyurethanes or polyurethanes having a bulk density of >500 kg/m3 are used in fields where not only the excellent materials properties but also antistatic or electrically conductive properties are important. Mention may here be made of floor coverings, tires, paintable rollers, rolls and embedding materials for electricals. Charges are to be avoided at all costs especially in some highly sensitive industrial equipment. Like most other polymers, polyurethanes are not conductive per se. Conventional surface resistances are in the region of 1013 ohm.
Numerous additives have been used to reduce this high resistance. Very early on, salts such as ammonium salts (e.g. Catafor® from Rhodia GmbH) were used to reduce the surface resistance. Unfortunately, these additives have the disadvantage of accelerating hydrolysis of polyurethanes based on polyester polyols. Furthermore, migration to the surfaces and, associated therewith, “chalking” is a great disadvantage. In addition, the effects achieved are comparatively small and the surface resistance is reduced by only 2-3 powers of ten.
Apart from the use of these salts, the use of conductive carbon black (for example conductive carbon black having a BET surface area of from 600 to 1200 m2/g; for example Ketjenblack® from Akzo Nobel Polymer Chemicals by) or of carbon fibers is also known. The use of conductive carbon black is described, for example, in EP-A 0 129 193 and DE-A 3 528 597. Conductive carbon blacks enable good surface resistances to be achieved in foamed and unfoamed polyurethanes (clown to 104 ohm). However, the amounts of conductive carbon black required always result in a very high viscosity of the reaction components, so that such systems can no longer be processed using conventional polyurethane machines. Such systems are therefore hardly used at all in industry. Significantly lower viscosities can be achieved by use of carbon fibers, as described in DE-A 19 858 825. Surface resistances below 104 ohm at just acceptable processing viscosities are achieved using relatively high concentrations of carbon fibers. However, in use it is found that in the case of mechanically stressed parts the fibers break and the conductivity decreases very quickly until a nonconductive polyurethane is obtained again. This breaking of the fibers occurs during processing, so that such PUR systems are not used in industry.
Furthermore, the use of graphites (e.g. Cond 8/96 from Graphit Kopfmühl AG) is conceivable for reducing the electrical resistance. However, to obtain a usable conductivity, concentrations which would mean α-considerable viscosity increase and therefore rule out industrial processing would be required in the polyurethane reaction system.
Disadvantages of the incorporation of carbon nanotubes are the difficulty of dispersing them and the high processing viscosity.